Medium access control protocol for ofdm wireless networks

ABSTRACT

A medium access contention protocol that is highly beneficial in wireless networks and particularly in wireless networks that employ a fixed minimum burst size such as OFDM wireless networks. In one embodiment, a MAC protocol is a demand-assigned protocol that maximizes utilization of the bus medium (the allocated frequency spectrum.) Each data communication device (DCD) in the network communicates with a central access point (AP). Multiple DCDs may request access from the AP in the same request access (RA) burst. Each of the multiple DCDs transmits its access request to the AP within a frequency domain channel in the RA burst that is orthogonal to the frequency domain channels used by the other DCDs requesting access. Each DCD includes channel training information in the access request burst to allow the AP and/or DCD to adapt to rapid variations in channel characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of co-assigned U.S. patentapplication Ser. No. 11/221,430 filed 8 Sep. 2005, titled MEDIUM ACCESSCONTROL PROTOCOL FOR OFDM WIRELESS NETWORKS. U.S. patent applicationSer. No. 11/221,430 is in turn a continuation of co-assigned U.S. patentapplication Ser. No. 09/738,061 filed Dec. 15, 2000, titled MEDIUMACCESS CONTROL PROTOCOL FOR OFDM WIRELESS NETWORKS, now U.S. Pat. No.7,020,069. U.S. patent application Ser. No. 09/738,061 is in turn acontinuation of co-assigned U.S. patent application Ser. No. 09/019,938filed Feb. 6, 1998, also titled MEDIUM ACCESS CONTROL PROTOCOL FOR OFDMWIRELESS NETWORKS, now U.S. Pat. No. 6,192,026. The contents of each ofU.S. patent application Ser. Nos. 11/221,430, 09/738,061 and 09/019,938are incorporated herein by reference.

The present application is related to the subject matter of aco-assigned application titled SPATIO-TEMPORAL PROCESSING FORCOMMUNICATION, U.S. patent application Ser. No. 08/921,633 filed on Aug.27, 1997, now U.S. Pat. No. 6,144,711, the contents of which are hereinincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to communication network protocols formedium access. In particular, the present invention relates tocommunication network protocols in the context of a wireless medium andin the context of communication networks that utilize fixed minimumpacket sizes.

Data communications devices (DCDs) on certain common types of networkmust share the communication medium. The function of a medium accesscontrol (MAC) protocol is to allow each DCD the opportunity to seize themedium to transmit its data according to the rules of the protocol. Inorder to facilitate effective data communications, the opportunities totransmit should occur such that the wait time between opportunities isnot excessive. In addition, access to the medium should be distributedfairly among the DCDs. A number of MAC protocols have been developed andfielded in wired networks.

These protocols include ALOHA, slotted-ALOHA, CSMA, and CSMA-CD. TheALOHA system is described in N. Abramson, “The ALOHA System—AnotherAlternative for Computer Communications,” 1970 Fall Joint ComputerConference, AFIPS Conference Proceedings, Vol. 37, AFIPS Press, MontvaleN.J., 1970, the contents of which are herein incorporated by reference.CSMA and CSMA-CD systems are described in Anthony S. Acampora, “AnIntroduction to Broadband Networks,” Plenum Press, New York, N.Y., 1994,the contents of which are herein incorporated by reference.

ALOHA and slotted-ALOHA are random access schemes that could be adaptedfairly easily to wireless networks. However, these MAC protocols sufferfrom poor maximum bus utilization.

While CSMA and CSMA-CD exhibit much better bus utilization, theseprotocols are much better suited to wired networks than wireless for thefollowing reason: The operation of both CSMA and CSMA-CD depend uponeach DCD in the network being able to sense when any of the other DCDsis transmitting. A DCD only transmits when it has determined that thebus is not currently in use by another DCD. This requirement becomesproblematic in a wireless network since it often occurs that not everyDCD in the network is within range of all the others.

FIG. 1 depicts a simple wireless network 100 with 3 DCDs 102. Bcommunicates with both A and C. A and C are separated by too large adistance to detect when the other is transmitting, and are thereforeobviously unable to communicate directly. To illustrate the problem thatcan arise, suppose A is transmitting to B. Since C cannot detect A'stransmissions, it will mistakenly assume that the medium is not beingused. Then, suppose that C, mistakenly believing that the bus is idle,attempts to transmit a message to B. As a result, a data collisionoccurs at B and the messages transmitted by both A and C are corruptedor one of the messages is lost. A situation such as this is commonlyreferred to as the “hidden terminal problem.”

FIG. 2 depicts a solution to the hidden terminal problem. A wirelessnetwork 200 includes several DCDs 202 and a specialized central DCD 204,also referred to as an access point (AP) 204. Each DCD 202 communicatesthrough AP 204. AP 204 allocates the use of the medium by all DCDs 202making up the network. In order to be integrated into the networkconfiguration, any remote DCD 202 must be within the coverage area of AP204. This ensures that DCD 202 is able to receive, and will thereforeadhere to, the commands issued by AP 204 concerning use of the medium.

MAC protocols using this network architecture have been implemented forcellular communication systems, wherein the base stations serve as APsand the cellular phones serve as the DCDs. However, because the natureof voice traffic is quasi-continuous and relatively low bandwidth,cellular MAC protocols are designed with circuit-switched channelassignments. The available spectrum is divided into frequency channelsand/or time slots and/or spread spectrum spreading code channels thatare assigned to a user for the duration of a call, regardless of whetherthere is any voice activity. This type of MAC protocol is inefficient ina typical computer or multi-media network due to the inherently burstynature of its traffic. Exchanging bursty traffic over a circuit-switchednetwork results in the circuit-switched connections frequently sittingidle.

With a demand-assigned protocol, usage of the bus is allocateddynamically by a bus arbiter according to the traffic demands of eachDCD on the network. One example of a demand assigned MAC protocol isDQRUMA which is described in Mark J. Karol, Zhao Liu, and Kai Y. Eng,“An Efficient Demand-Assignment Multiple Access Protocol for WirelessPacket Networks”. ACM/Baltzer Wireless Networks, Vol. 1, No. 3, pp.267-279, 1995, the contents of which are herein incorporated byreference. Under this protocol, each DCD that has data to transmitnotifies the AP. Any DCDs needing to use the bus submit their requestsduring a predefined, regularly reoccurring, time period called therequest access (RA) slot. Whenever more than one DCD submits a requestduring the predefined period, all those requests are lost in acollision. In effect, the access request process operates like aslotted-ALOHA system, i.e., time-aligned random-access transmissions.

Upon receiving a valid access request, the AP sends back anacknowledgement message, and places the terminal's ID in a queue withother DCDs whose access requests were received but that have not yetbeen able to complete their transmissions. The AP manages the queueaccording to any one of many possible assignment algorithms. The APnotifies a given DCD shortly before its turn to use the bus. The DCDthen uses the bus for a fixed, and reasonably short, period of time. Ifthe DCD hasn't finished transmitting all of its data at the end of itsallotted bus access period, it tacks a “piggyback request” onto the endof its transmission. The piggyback request lets the AP know that the DCDthat just finished transmitting needs the bus again. This is equivalentto submitting a contention-free access request, helping to completetransfers which have already started. In addition, the piggyback requestscheme significantly reduces the number of DCDs contending for access inthe RA slot.

A demand-assigned protocol such as DQRUMA possesses many desirablefeatures for a wireless data network as has just been described.However, it also possesses several undesirable qualities making itdifficult to implement on many wireless networks. For instance, DQRUMAassumes the existence of simultaneous parallel uplink (traffic goinginto the AP) and downlink (traffic coming out of the AP) channelsbetween the AP and the DCDs. If the parallel channels each have equalcapacity, the bus can only operate at maximum efficiency when trafficinto and out of the AP is perfectly balanced between uplink anddownlink. Whenever the traffic is not balanced, one of the channels mustoperate below capacity. It is difficult, if not impossible, toreallocate bandwidth between the uplink and downlink channels inresponse to varying loads.

The only practical way to obtain two simultaneous channels in a wirelesssystem is through frequency division duplexing (FDD), i.e., uplinktraffic resides on one carrier frequency and downlink traffic resides onanother. Often, frequency spectrum allocations for a given applicationdo not lend themselves to the implementation of FDD systems. Unless theuplink and downlink frequency bands can be separated (intonon-contiguous blocks) the analog filtering (diplexer) requirements forthe wireless transceiver become extremely difficult if one is to avoidwasting a large portion of the spectrum.

In DQRUMA, requests for access to the bus, and the acknowledgements ofthose requests, are relatively short messages. The DQRUMA protocol isdesigned to use short requests and acknowledgement messages and longerdata packets. However, in certain systems, such as networks that employOFDM (Orthogonal Frequency Division Multiplexing), it is difficult tovary the size of the message bursts. Unless the data bursts in thesystem are very small, using the same size bursts to transmit accessrequests and acknowledgements will result in a many unused data bit inthose bursts, adversely impacting spectral efficiency.

Furthermore, once a DCD has received an acknowledgement of its accessrequest, it must continually listen to messages from the AP as it waitsits turn to use the bus. This is a significant disadvantage for portablewireless DCDs, where battery life is a major consideration.

A medium access protocol for wired networks has been proposed in whichmultiple DCDs transmit overlapping messages during a single OFDM burstin such a way that the AP correctly receives each of the individualmessages. See K. S. Jacobsen, J. A. C. Bingham, and J. M. Cioffi, “ADiscrete Multitone-based Network Protocol for Multipoint-to-pointDigital Communications in the CATV Reverse Channel,” in 1995 CanadianCable Television Association (CCTA) Technical Papers, May 1995, thecontents of which are herein incorporated by reference. However, forthis method to work properly, the AP must have knowledge of the channelbetween itself and each DCD transmitting the message. To obtain thischannel knowledge, a separate channel training routine is executed, withthe AP storing the channel measurements for later use. This is aworkable solution for the time invariant (or very slowly varying) cabletelevision channels contemplated by Jacobsen, et al.

However, in a wireless network the channel changes so rapidly that eachmessage transmitted by a DCD propagates through an essentially unknownchannel before reaching the AP. The Jacobsen, et al. method is thusunusable in the wireless context.

What is needed is a MAC protocol that efficiently accommodates fixedminimum packet sizes in a wireless contexts and that furthermore allowsthe DCD to deactivate its idle circuitry during bursts in which itneither transmits nor receives.

SUMMARY OF THE INVENTION

The present invention provides a medium access contention protocol thatis highly beneficial in wireless networks and particularly in wirelessnetworks that employ a fixed minimum burst size such as OFDM wirelessnetworks. In one embodiment, a MAC protocol according to the presentinvention is a demand-assigned protocol that maximizes utilization ofthe bus medium (the allocated frequency spectrum.) Each datacommunication device (DCD) in the network communicates with a centralaccess point (AP). The AP is responsible for assigning usage of the bus.

Multiple DCDs may request access from the AP in the same request access(RA) burst. Each of the multiple DCDs transmits its access request tothe AP within a channel in the RA burst that does not interfere withother channels used by the other DCDs requesting access. Each DCDincludes channel training information in the access request burst toallow the AP and/or DCD to adapt to rapid variations in channelcharacteristics. Once the AP has determined and distributed a schedulefor access requests, individual DCDs may idle themselves during periodswhen they are scheduled for either transmission or reception.

A first aspect of the present invention provides a method for sharingaccess to a common wireless medium in a wireless communication system.The method includes a step of transmitting an access request burst froma plurality of data communication devices to a central access point. Theaccess request burst is divided into a plurality of OFDM tones. Each oneof the plurality of data communications devices transmits using only asubgroup of the plurality of OFDM tones. The method further includes astep of receiving the access request burst at the central access point.In response to receipt of the access request burst at the central accesspoint, the plurality of data communication devices are scheduled foraccess to the common wireless medium.

A second aspect of the present invention provides a method for operatinga wireless local area network. The method includes steps of defining aburst length for communication, and transmitting bursts having the fixedburst length from a plurality of data communication devices coupled tothe wireless local area network. The bursts include bursts carrying dataand bursts carrying access request information.

A further understanding of the nature and advantages of the inventionsherein may be realized by reference to the remaining portions of thespecification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a prior art wireless network of data communicationdevices.

FIG. 2 depicts a wireless network of data communication devices and acentral access point that may be operated in accordance with oneembodiment of the present invention.

FIG. 3 depicts an OFDM communication system that may be operated inaccordance with one embodiment of the present invention.

FIG. 4A depicts portions of a data communication device or centralaccess point according to one embodiment of the present invention.

FIG. 4B depicts an air interface frame according to one embodiment ofthe present invention.

FIG. 5 depicts an information burst according to one embodiment of thepresent invention.

FIG. 6 depicts an access request burst according to one embodiment ofthe present invention.

FIG. 7 depicts further details of an access request burst according toone embodiment of the present invention.

FIG. 8 depicts a permissions and acknowledgments burst according to oneembodiment of the present invention.

FIG. 9 depicts a superframe including multiple air interface framesaccording to one embodiment of the present invention.

FIG. 10 depicts details of a special request access burst according toone embodiment of the present invention.

FIG. 11 depicts further details of a special request access burstaccording to one embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS Exemplary Wireless System

The explanation of the present invention will assume an exemplarywireless system. Like the system depicted in FIG. 2, the exemplarywireless system includes access point 204 and multiple datacommunication devices 202. Data communication devices (DCDs) 202 andaccess point (AP) 204 each employ a orthogonal frequency divisionmultiplexed (OFDM), radio modem to receive and transmit over the networkmedium. The shared network medium, also referred to as “the bus”, is, inthis case, the frequency spectrum allocated for the wireless network.

FIG. 3 depicts an OFDM communication system 300 that may be operated inaccordance with one embodiment of the present invention. OFDMcommunication system 300 represents a combination of the transmitterside of the modem at a DCD and the receiver side of the modem at the AP,and the channel in between them. A comparable system operates on thedownlink from the DCD to the AP.

A symbol mapper 302 maps data bits to M-ary QAM frequency-domainsymbols, where M=2^(b) and b is the number of bits per symbol. The QAMconstellation used in the exemplary embodiment is 4-QAM in which eachsymbol represents 2 data bits. The number of symbols per OFDM burst ischosen to be N=2^(n), where N=256 in the exemplary system. The N symbolsare converted to a 256-point time-domain sequence through an inverseFast Fourier Transform (IFFT) operation by an IFFT stage 304. A cyclicprefix application stage 306 copies the last v points in the time-domainsequence and affixes the copies to the beginning of the sequence,increasing the length of the time-domain sequence to 256+v points. Thisoperation is known as the addition of a “cyclic prefix.” The motivationfor employing a cyclic prefix is discussed below.

The complex digital time-domain sequence is converted to in-phase (I)and quadrature (Q) analog signals that are used to quadrature-modulatean RF carrier. The RF carrier is then radiated from a transmitterantenna. At the receiver, the signal is captured by another antenna anddownconverted into I and Q baseband analog waveforms, that are thenconverted to digital representations with analog-to-digital converters.For simplicity, in this discussion we will consider an equivalentcomplex baseband channel 308 modeled as a finite impulse response (FIR)system. Adopting the baseband system model greatly simplifies thediscussion. With this model, the output of the channel is given by theconvolution of the complex time-domain samples with the channel impulseresponse, h. The length of the baseband channel is limited to L symbols,and the FIR channel is described by the channel coefficients {h₀, h₁, .. . , h_(L-1)}. The channel length corresponds to the time span overwhich the multipath energy arrives at the receiver due to an impulsebeing transmitted by the transmitter.

The receiver captures the output of the channel. A separate timingadjustment circuit 310 is used to determine the first sample of eachincoming burst. The cyclic prefix is removed by a cyclic prefix removalstage 312, and the remaining time-domain samples are fed into a256-point FFT stage 314 that produces a 256-point received frequencyspectrum. Given knowledge of the frequency-domain channel response H(f),each point in the received frequency-domain sequence can be correctedfor the channel's amplitude scaling and phase shift at that frequency bya symbol detection stage 316. A channel estimation circuit 318 usesknown training tones that are embedded in each burst to recover thecomplex scalars (one for each of the 256 frequency bins) describing thechannel frequency response. A discussion of channel estimation methodsis found in the cross-referenced SPATIO-TEMPORAL PROCESSING FORCOMMUNICATION application. Once the received frequency-domain data iscorrected for the channel response, the constellation point closest toeach sample is determined by symbol detection stage 316. Finally, usingthe same bits-to-symbol mapping as the transmitter, the output bits areproduced by a bit mapper 320. It is understood that coding may also beemployed in OFDM communication system 300.

The purpose of the cyclic prefix is to “orthogonalize” the channel. Thatis, it prevents energy from any frequency-domain symbol from interferingwith any other frequency-domain symbol, even in frequency-selectivechannels as described in A. Peled and A. Ruiz, “Frequency domain datatransmission using reduced computational complexity algorithms,” IEEEInt. Conf. Acoust., Speech, Signal Processing, Denver, Colo., April1980, pp. 964-967, the contents of which are herein incorporated byreference. In order for the orthogonalization to hold, the length of thecyclic prefix must equal or exceed the length of the channel, v≧L. Thechannel length is a function of the operating environment, with indoorchannels typically being much shorter than outdoor channels. Forexample, in an office environment the duration of the impulse responseis typically less than 2 microseconds. In large outdoor wirelesssystems, channels can reach lengths of 25 microseconds or more.

Once again considering the office environment, assuming that the samplerate of the OFDM system is 10 MHz, then the length of the channel is 20samples. The requirement stated above regarding the minimum length ofthe cyclic prefix assumes perfect synchronization of the incomingsignal. If any timing uncertainty exists, as it does with any practicalsystem, the length of the cyclic prefix can be increased to account forthe maximum timing error. Assume that the timing error is ±3 samples.Then the length of the cyclic prefix must be ≧26 samples.

The maximum number of data bits per transmission is given by b·N=512.The actual number will be lower because a number of the Nfrequency-domain symbols are reserved for channel training tones. ManyMAC protocols for wired systems use short transmissions for busmonitoring and control. In a wireless network one might also desire theability to send brief transmissions for monitoring and control whilemaintaining the ability to transmit longer bursts for data. A schemesuch as this tends to optimize efficient use of the bus resource. Ofcourse when sending fewer data bits per burst, a corresponding shorterduration transmission is expected. In an OFDM system, this isinconvenient since sending shorter and longer bursts requires both thetransmitter and receiver to implement (I) FFTs of different lengths. Thepresent invention provides a system and method for sending briefmessages in an efficient manner and one which does not require more thana single-length (I) FFT implementation.

Top Level MAC Protocol and Apparatus

FIG. 4A depicts portions of a data communication device or centralaccess point according to one embodiment of the present invention. AnOFDM modem 322 includes the transmitter and receiver circuitry discussedwith reference to FIG. 3. A MAC processor 324 is responsible forimplementing a MAC protocol as will be described. The operation of MACprocessor 324 will of course depend on whether it is incorporated withina DCD 202 or within AP 204. MAC processor 324 exchanges protocol-relatedinformation with OFDM modem 322. A power control processor 326,typically incorporated only within DCD 202, turns off portions of DCD202 when no access to the wireless medium is scheduled as determined bythe operation of MAC processor 324.

Preferably, central AP 204 controls usage of the network medium. Byrequiring DCDs 202 to synchronize to AP 204 prior to transmitting anysignals, the hidden terminal problem is eliminated.

A top level schedule for use of the medium by DCDs 202 and AP 204 isdefined by a MAC protocol data structure referred to as an Air InterfaceFrame. FIG. 4B depicts an AIF 400 according to one embodiment of thepresent invention. AP 204 transmits a predetermined waveform during aSynchronization (SYNC) burst 402. Each DCD 202 uses SYNC burst 402 forfrequency and timing acquisition. SYNC burst 402 is a physical layerburst.

A Permission and Acknowledgement (P&A) burst 406 follows SYNC burst 402.After P&A burst 406, there are 32 INFO bursts 408 carrying data. Thefunction of P&A burst 406 is twofold: to acknowledge which accessrequests were received in a previous RA burst 404, and to broadcast theassignment of each INFO burst 408 within the current frame 400. P&Aburst 406 is a MAC layer burst while INFO bursts 408 are physical layerbursts.

During Request Access (RA) burst 404, any of DCDs 202 needing to use thebus may transmit an access request. Since DCDs 202 submit accessrequests without coordination, the possibility of collisions existswithin RA burst 404. The present invention provides multiple channelsfor simultaneous access requests during RA burst 404. This reduces theprobability of collisions and makes more access request opportunitiesavailable without reducing efficiency. RA burst 404 is a MAC layerburst.

When a DCD 202 initially contacts AP 204, AP 204 responds with a timingadjustment command. This is required because, even though DCD 202 hassynchronized itself to the AP 204's transmissions, range uncertainty cancause messages transmitted by different DCDs to overlap upon arrival atAP 204. It is advantageous to set aside initial access intervals inwhich poorly aligned transmissions from DCDs 202 that haven't yetadjusted their timing do not interfere with other messages on the bus.Additional guard time can be used in conjunction with selected RA bursts404 to accommodate the range uncertainty associated with DCDs 202initially accessing the network. The approach described herein resultsin increased efficiency because the parallel RA channels all share theadded guard time. In a conventional system, additional guard time isrequired for each RA channel.

After RA burst 404, AP 204 transmits by communicating the usage of INFObursts 408 within the frame in advance, those DCDs 202 not scheduled totransmit or receive during the frame may disable unused circuitry untilthe next SYNC burst 402. Disabling idle circuits is a valuable strategyto extend battery life in portable DCDs 202.

This MAC protocol is designed to operate over a single bi-directionalbus the usage of which is divided into multiple bursts. Unlike manywireless MAC protocols, the MAC protocol described herein does nottypically require that information emanating from AP 204 be transmittedin one portion of the spectrum, and information flowing into AP 204 betransmitted in another portion of the spectrum. This results in AP 204being able to allocate usage of the medium such that the bus utilizationremains high even when the AP traffic is asymmetrical (with respect tothe AP).

Thus, AIF 400 includes 35 bursts. Of the 35 bursts, one burst isdedicated to physical layer functions, 32 bursts are dedicated to datalink layer functions, and two bursts are dedicated to MAC layerfunctions. The 32 Information (INFO) bursts 408 may be transmitted byeither AP 204 or a particular DCD 202 and carry primarily payload. Usageof INFO bursts 408 is controlled by AP 204 in response to varyingtraffic loads on the bus.

In RA burst 404, P&A burst 406, and INFO bursts 408, a certain number ofsymbols are reserved for training and carry no payload data. For channelestimation, the number of symbols used for training should equal orexceed the length of the longest channel in which the system is expectedto operate. In a preferred embodiment, 32 frequency-domain symbols areset aside for training, leaving 224 frequency domain symbols (448 bits)to carry information in each burst. The 448 bits in MAC-layer and INFObursts are divided amongst several fields.

Internal Burst Structure

FIG. 5 depicts an INFO burst 408 according to one embodiment of thepresent invention. INFO burst 408 includes 4 major fields: HDR 502, Data504, PGBK506, and FEC 508. The 11-bit HDR (header) field 502 carriesdata link layer information including the message type and messagesequencing information. The 384-bit Data field 504 carries user payload.The 5-bit PGBK field 506 is used to implement the MAC-layer Piggybackfunction. With this field, a DCD 202 informs AP 204 whether or not ithas additional data to transmit. DCD 202 writes the number of additionalbursts it needs (from 1 to 31, in binary representation) in PGBK field506. DCD 202 writes 5 zeros into PGBK field 506 to tell AP 204 that ithas no pending data to send once the current transmission is finished.The last field in INFO burst 408 is FEC (forward error correction) field508. The 6-byte FEC field 508 carries redundant information derived fromthe other 50 bytes in the burst. The redundant information is exploitedto correct data errors when they occur in transmission. The FEC codingemployed could be any one of several techniques, however a byte-basedReed-Solomon code is a very efficient error-correction technique for a448-bit codeword. In the exemplary embodiment, 6-byte FEC field 508allows up to 3 incorrectly received bytes within the burst to becorrected.

FIG. 6 depicts RA burst 404 according to one embodiment of the presentinvention. RA burst 404 contains 190 bits that are divided into 5identical fields 602. In this exemplary embodiment, up to 5 DCDs 202 cansubmit 38-bit access request messages during RA burst 404. Each of theparallel access request opportunities within a burst is referred to asan “RA channel.” The unique design of RA burst 404 enables simultaneousaccess attempts to be heard by AP 204 thereby reducing contention amongnetwork DCDs 202. Furthermore, the design allows a burst large enough tocarry data to be used for bus control functions without sacrificingefficiency and preserving the use of a single-length (I) FFT throughoutthe system.

In an OFDM system, the tones within a burst are substantiallyindependent from one another, i.e., no inter-carrier interference isproduced. This property is exploited to create the parallel RA channelswithin the RA burst. This is accomplished by dividing the burst's tonesinto 5 mutually exclusive subsets. Each subset of tones constitutes oneRA channel. Those tones that are not assigned to a given RA channel arenot energized by the DCD using that particular RA channel. That is, afrequency-domain symbol of zero magnitude is transmitted on those tonesbelonging to all other RA channels.

FIG. 7 depicts the division of the tones of RA burst 404 into the fivechannels 602. The tones associated with each RA channel 602 are made upof data tones (D_(i)) and training tones (T_(i)), where i denotes the RAchannel number. Tones with zero energy are labeled with “0”. Forsimplicity, only the first and fifth RA channels are shown.

While the data tones may be grouped in virtually any manner deemedconvenient, for best performance the training tones should span the OFDMburst and be spaced at constant intervals if the minimum number oftraining tones is to be used. FIG. 7 assumes that 32 tones are requiredto characterize the channel. The i^(th) RA channel uses tones n=i+8k,k=0, 1, . . . , 31, for training. The available data tones are assignedto the 5 RA channels in sequential fashion. Note that because the 256tones in the RA burst do not divide equally into the 5 RA channels 602,tone number 256 is unused. Each RA channel 602 includes 19 data tonesthat carry information with 2 bits per tone. This configuration resultsin a 38-bit RA channel.

With this approach, 5 DCDs 202 each using a different RA channel 602 cantransmit simultaneously during the RA burst without interfering with oneanother. The AP processes the RA burst in the usual fashion up throughFFT stage 314. Then the frequency domain tones are separated into the 5sets of tones corresponding to the 5 RA channels. Using the i^(th) setof training tones (T_(i)), the channel response vector h thatcorresponds to the i^(th) RA channel is estimated separately. Thechannel response estimate for the i^(th) channel is applied to the setof RA channel data tones (D_(i)) to recover the data bits that were senton the i^(th) RA channel. This process is repeated for i=1, 2, . . . ,5.

This burst sharing technique is thus applicable to time-varying channelssuch as wireless channels because the training tones are distributedthrough each access request burst. Since access requests are generatedrandomly by the DCDs, the most efficient way to perform the channeltraining is to embed training tones in each RA channel as has beendescribed herein.

Referring once again to FIG. 6, observe that each of RA channel fields602 is divided up into 3 sub-fields: an RID sub-field 604, a BDsub-field 606, and an FEC sub-field 608. RID sub-field 604 is used by aDCD 202 to distinguish itself as the originator of the RA submission.Each DCD 202 fills this sub-field with an identification number whentransmitting in an RA channel 602. In one possible embodiment, a networkadministrator configures each DCD 202 that will connect to the networkby assigning each a unique 17-bit ID number. Using 17 bits allows for131,072 unique ID numbers to be used in a network.

BD sub-field 606 is used by DCD 202 to indicate the number of burststhat it wishes to use. The 5-bit field can be used to specify thedesired number of bursts from 1 to 32. For example, suppose a DCD 202has 300 bytes of data in its transmit buffer. This amount of data spans6.25 INFO burst data fields. So, when submitting its access, the DCDwould fill BD sub-field 606 with 00111 (binary 7).

FEC sub-field 608 contains redundant information based upon tRID and BDsub-fields 604 and 606. It is used to correct bit errors that may occurin transmission. With the small number of bits in each RA channel abit-based cyclic redundancy check (CRC) coding scheme is appropriate,though other coding schemes could also be used. RA channels 602 aredesigned to contain a large amount of FEC redundancy to ensure robustoperation of MAC layer functions.

FIG. 8 depicts P & A burst 406 according to one embodiment of thepresent invention. Preferably, only AP 204 is permitted to transmitduring P&A burst 406 and its function is twofold. AP 204 uses this burstto acknowledge access requests received successfully during previous RAburst 404. In addition, AP 204 assigns the usage of INFO bursts 408during the present air interface frame 400.

The first five fields 802 of P&A burst 406, ACK 1 through ACK 5, areused to acknowledge the reception of access requests from DCDs 202 whichwere received during previous RA burst 404. The ACK fields 802 eachcontain two sub-fields. The DCD's ID number (that which the DCD enteredinto RID field 604 of RA channel 602) is placed into a 17-bit AIDsub-field 804. A DCD 202 awaiting acknowledgment of an access requestmonitors AID sub-fields 804 in P&A burst 406 looking for an entry thatmatches its own ID number. Reception of an ACK field 802 with a matchingID sub-field 804 serves as a positive acknowledgment of the DCD's accessrequest. A TID sub-field 806 is used to carry a 9-bit temporaryidentification (TID) number. This number is used by one DCD 202 for theduration of an access event in which it is involved. An access event isdefined to be the time from which an RA is successfully received to thetime when a DCD 202 has finished sending its last burst. The last burstis sent after any piggyback requests are submitted and cleared, whichcan result in relatively long access events. The 9-bit TID sub-field 806allows up to 512 access events to exist simultaneously. When a DCD 202receives an RA acknowledgment, it records the number given in TIDsub-field 806 and stores it for use throughout the duration of theaccess event.

The ACK fields 802 are followed by a downlink transmissions (DLTX) field808 and eight permission-to-transmit (PTX) fields 810. The 3-bit DLTXfield 808 designates the number of PTX fields 810 that correspond todownlink transmissions, beginning with PTX1. For example, if thecontents of DLTX are [011], then PTX1, PTX2, and PTX3 refer to downlinktransmissions while PTX4, PTX5, PTX6, PTX7, and PTX8 refer to uplinktransmissions. Each PTX field 810 includes 4 sub-fields: TXID 812, NUM814, PWRC 816, and DLY 818. TXID sub-field 812 specifies the DCD 202involved in a given burst. The TXID number in PTX field 810 musttherefore match the TID number supplied to the DCD 202 in a prior ACKfield 802. If the PTX field 810 is one of the first n (where n is thenumber inserted into DLTX field 808) within P&A burst 406, then TXIDsub-field 812 specifies the DCD 202 scheduled to receive a downlinktransmission. If the PTX field 810 is not one of the first n, then theTXID sub-field 812 specifies the DCD 202 that is to transmit an uplinkmessage. NUM field 814 specifies the number of consecutive INFO bursts408 to be received (or transmitted) by DCD 202 specified in TXIDsub-field 812. In order to illustrate the function of P&A burst 406,consider the example given in Table 1 and Table 2. The values in thepertinent fields in an exemplary P&A burst 406 are given in Table 1. Thecorresponding usage of the frames INFO bursts is given in Table 2.

TABLE 1 SELECTED FIELD ENTRIES FROM AN EXEMPLARY P&A BURST. Field(Sub-Field), Value (Sub-Field), Value DLTX 3 PTX 1 TXID, 4 NUM, 2 PTX 2TXID, 473 NUM, 10 PTX 3 TXID, 85 NUM, 3 PTX 4 TXID, 62 NUM, 5 PTX 5TXID, 4 NUM, 12 PTX 6 TXID, 0 NUM, 0 PTX 7 TXID, 0 NUM, 0 PTX 8 TXID, 0NUM, 0

TABLE 2 CORRESPONDING INFO BURST USAGE. INFO Burst Number DCD TID NumberDCD State 1, 2 4 Receiving  3-12 473 Receiving 13-15 85 Receiving 16-2062 Transmitting 21-32 4 Transmitting

The design of P&A burst 406 permits a DCD 202 to determine preciselywhether it will be involved in the exchange of each INFO burst 408 inAIF 400. PWRC sub-field 816 is used to facilitate power control. This2-bit sub-field 816 is used to instruct the DCD 202 specified in the IDsub-field to increment, decrement, or leave unchanged its RF outputpower. Likewise, DLY sub-field 818 is used to instruct the DCD toadvance, retard, or leave unchanged, the timing of its bursttransmissions.

If eight PTX fields 810 are not sufficient to schedule all 32 bursts inAIF 400, then a P+ bit 820 is asserted. This indicates that a second P&Aburst 406 immediately follows the usual P&A burst, thereby supplantingthe first INFO burst 408. A 2-bit RSVD field 822 that is reserved forfuture use follows the P+ field. A last field (FEC) 824 contains 96forward error correction bits. A large number of FEC bits are includedin P&A burst 406 to help ensure the integrity of the MAC layeroperations which are critical to robust system operation.

The structure of P&A burst 406 enables implementation of a “sleep mode”feature. After receiving and decoding P&A burst(s) 406, each DCD 202knows during which, if any, of INFO bursts 408 within AIF 400 it will bereceiving or transmitting. For those bursts in which a DCD 202 will beinactive, power control processor 326 powers down the circuitry involvedin burst reception and transmission. Power control processor 326continues keeping time and reactivates the unit for reception of thenext frame's SYNC and P&A bursts 402 and 406. While certain circuits,such as interface and clock circuits must operate continually, the sleepmode feature can substantially reduce the DCD's power consumption.

FIG. 9 depicts a modified frame structure to accommodate DCDs 202 with alarge amount of timing uncertainty according to one embodiment of thepresent invention. The large timing uncertainty will generally beassociated with DCDs 202 that are contacting the AP for the first timein a session. In all subsequent transmissions the DCDs 202 will adjusttheir timing with high accuracy in response to a command from AP 204.Therefore, a superframe structure is used to accommodate these initialaccesses by DCDs 202. A superframe 900 includes a predetermined number(20 in the example) of AIFs, with a special first AIF 902 in superframe900 having special characteristics. The receiver of DCD 202 candetermine the position of first frame 902 within superframe 900 bysearching for a unique marker within that frame's first burst, an XSYNCburst as described below.

The special first AIF 902 in the superframe includes the unique burstsXSYNC 904 and XRA 906 that replace the SYNC burst and RA burst. XSYNCburst 904 is a modified version of the previously described SYNC burst.It still permits the DCD to obtain frequency offset correction and bursttiming, yet it is constructed so that it can be uniquely identified asthe XSYNC burst. XRA burst 906 is like the regular RA burst, but withadditional cyclic prefix samples, additional training tones, and a smallguard time interval. The XRA burst's modified structure allows thesystem to ascertain the time of arrival of each parallel unsynchronizedaccess request, but prevents these access requests from overlapping thenext burst in the superframe. XRA burst 906 remains compatible with thesystem's 256-point I(FFT) convention.

Recall that previously it was assumed that the DCD transmit timinguncertainty was insignificant; either the distance between DCDs and theAP was sufficiently small, or the DCD adjusted the timing of itstransmissions per instructions from the AP. Now suppose that 500 metersseparate the AP and the farthest DCD. If the DCD has not previouslycontacted the AP, then it has not yet received a timing adjustmentcommand. Thus if the DCD were to submit an access request during aregular RA burst according to the timing it perceives, its transmissionwould arrive at the AP roughly 3.2 microseconds too late in the framestructure. In an exemplary embodiment of the XRA slot, we consider aconstruction that allows for roundtrip time delays, and therefore burstmisalignments of up to 3.2 microseconds. In the exemplary system withthe 10 MHz sample rate, this corresponds to 32 samples.

The timeslot of XRA burst 906 is sandwiched between the prior andsubsequent bursts 1002 in FIG. 10. The 32 samples of timing misalignmenteffectively lengthen the channel to L_(X)=L+32=52 samples. Recall thatboth the length of the cyclic prefix and the number of training tonesshould be greater than or equal to the length of the channel, i.e.,v_(X)≧L_(X) and N_(XT)≧L_(X). In the example, v_(X)=N_(XT)=64 samples ischosen. In addition, a guard time of at least 32 symbols must beincluded in the superframe timing to prevent late arriving XRAtransmissions from overlapping the next burst.

With the increased number of required training tones for access attemptswithin XRA burst 906, the total number of tones may no longer be dividedinto 5 sets as in the regular RA burst. Instead XRA burst 906 allowsthree parallel XRA channels. Each of the channels includes 64 trainingtones and 21 data tones. However, to remain consistent with the regularRA channels, one may choose to use only 19 of the 21 data tones in eachXRA channel. FIG. 11 depicts the arrangement of tones within XRA burst906.

The advantage of this approach is that up to 3 DCDs making initialaccess request can transmit during the XRA burst and not interfere withone another nor with communications taking place in adjacent bursts. Theslight overhead penalty of 64 samples per superframe enables up to 3initial access attempts.

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art and areto be included within the spirit and purview of this application andscope of the appended claims. All publications, patents, and patentapplications cited herein are hereby incorporated by reference.

1. A method comprising: wirelessly receiving at a base station of awireless communication system a plurality of access request bursts froma respective plurality data communication devices, each access requestburst being divided into a plurality of OFDM tones, the receiving as aresult of each one of said plurality of data communications devicestransmitting using only a subgroup of said plurality of OFDM tones; inresponse to the wirelessly receiving, scheduling said plurality of datacommunication devices for access to a common wireless medium to developan access schedule, wherein each said subgroup of said access requestburst comprises both tones carrying access request information andpredetermined channel training tones.